1. Field of the Invention
The present invention relates to a data receiving system for receiving frequency shift keyed (FSK) signals which are applied to a direct-conversion receiver of radiocommunication system.
2. Description of the Prior Art
Recently, a direct conversion receiver using FSK signals of a radio-frequency carrier wave has been examined if it is suitable as a receiver for realizing an integrated circuit.
For example, there is known a system disclosed in the Japanese Laid-open Patent Application No. SHO 58-81363.
Hereinafter, with reference to FIG. 19, a conventional FSK data receiving system is simply explained.
In FIG. 19, in the case where a reference symbol fc denotes a carrier wave frequency, and a reference symbol .DELTA.f denotes an FSK-modulated frequency deviation, an antenna 121 receives an FM input signal of frequency fc.+-..DELTA.f. And then, the received signal is inputted into one input terminal of a mixer 122. An another channel signal adjacent to this desired signal is also received by the antenna 121 and sent to the mixer 122.
The other input terminal of the mixer 122 is connected to a highly-stabilized local oscillator 120 which outputs a signal having a frequency f.sub.L =fc-.delta.f. That is, the frequency f.sub.L is offset from the carrier wave frequency fc by a small amount (.delta.f), though f.sub.L is within a signal channel. The output signals of the mixer 122 include signal frequencies .DELTA.f+.delta.f, .DELTA.f-.delta.f, and frequency-deviated adjacent channel signals.
Since the peaks between two signal frequencies .DELTA.f+.delta.f and .DELTA.f-.delta.f are spaced by an amount of 26, these signals can be separated by use of a proper discriminator. Subsequently, the adjacent channel signals are eliminated by a low-pass filter 123. The two signal frequencies are then mutually separated by bandpass filters 125 and also separated from all the low-frequency.
Output signals from respective bandpass filters 124, 125 are, in turn, inputted into their amplitude (or envelop) detectors. In order to reproduce the data signal, output signals of the amplitude detectors 126, 127 are compared in a difference circuit 128. Thus, data output can be obtain from a terminal 129.
However, in -this conventional system, since only one frequency mixer of local oscillator frequency band is used for receiving FSK data, a local oscillator frequency must be stable. For this reason, a large-size and expensive high-stable local oscillator is required, or an automatic frequency control (AFC) circuit needs to be installed on a local oscillator.
An AFC circuit requires another oscillator or a frequency mixer. Furthermore, in a quadrature demodulator circuit, two frequency mixers of local oscillator frequency band are required, Therefore this is not suitable for realizing a small-size, low power consumption, FSK data receiving system.
Though this quadrature demodulator circuit has a relatively large allowable width against a frequency offset of local oscillator, it is not still satisfactory. The conventional demodulation system was suffered from above-described problems.
Furthermore, as a conventional demodulation system, there is known, for example, a system disclosed in the Japanese Laid-open Patent Application No. SHO 58-19038. Hereinafter, with reference to FIG. 20, this conventional FSK data receiving system is simply explained.
In FIG. 20, in the case where a reference symbol fc denotes a carrier wave frequency, and a reference symbol .delta. denotes an FSK-modulated frequency deviation, a receiving RF signal of frequency fc.+-..delta. is on one hand directly applied to a first mixer circuit 181 and on the other hand applied to a second mixer circuit 182 through a phase shifter 183. The phase shifter 183 causes a 90-degree phase deviation at the carrier wave frequency Fc. A local oscillator 184 which is operated at the carrier wave frequency fc has two outputs supplied to the two mixer circuits 181 and 182, respectively.
Outputs from the two mixer circuits 181 and 182 pass low-pass filters 185,186, respectively. Output signals from these low-pass filters 185, 186 have a frequency difference between the input signal and the local oscillator. Then, a second phase shifter 187 gives the output from the low-pass filter 186 a 90-degree phase deviation at the base-band signal frequency.
Both signals are supplied into limiting amplifiers 188 and 189, respectively. Outputs from the limiting amplifiers 188 and 189 are dealt with as digital signals and processed in the digital logic network 190. By the way, the phase shifter 183 can be disposed between the local oscillator 184 and the first mixer circuit 181 or the second mixer circuit 182, instead of the disclosed position.
However, with above constitution, if there is an offset between the local oscillator frequency and the modulated carrier wave frequency, though one base-band signal frequency is increased in accordance with frequency difference between an upper and a lower frequencies of the FSK-modulated frequency signal, the other base-band signal frequency is decreased. Therefore, a 90-degree phase shifter which usually requires a large chip and has a wide band at base-band is required for realizing an integrated circuit. Especially, PG,6 error-rate of data demodulation is largely deteriorated due to the base-band signal being thus lowered. In case of high-speed data transmission, since an equivalent modulation index becomes small, its deterioration is further worsened.
Still further, there have been proposed another conventional direct-conversion receivers. FIG. 21 shows a system for demodulating from the phase relationship between two quadrature channels. (Refer to, for example, the national convention 2232 of the Electronics Information Communication Institution, spring 1987) FIG. 22 shows a system for causing a slight offset between the local oscillator signal frequency and the carrier wave signal frequency to make a demodulation based on a frequency difference of its output signal. (For example, the Japanese Laid-open Patent Application No. SHO 60-237749)
In FIG. 21, a reference numeral 101 denotes an antenna, and a reference numeral 102 denotes a low-noise amplifier circuit. Reference numerals 103, 104 are mixer circuits, and a reference numeral 105 denotes a quadrature circuit. A reference numeral 106 denotes a local oscillator, and reference numerals 107, 108 denote low-pass filters. Reference numerals 109, 110 denote limiter amplifiers, and a reference numeral 111 denotes a sign judging circuit. Furthermore, in FIG. 22, a reference numeral 112 denotes a frequency discriminating circuit, and a reference numeral 113 denotes a sign judging circuit.
In the direct-conversion receiver constituted as described above, its operation is explained hereinafter. First of all, in the quadrature demodulation system of FIG. 21, the local oscillator signal is divided into two quadrature signals in the phase shifter 105. These divided signals are mixed with the carrier wave signal in the mixer circuits 103,104. Baseband signals outputted from the mixer circuits are sent to the low-pass filters 107, 108 and the limiter amplifiers 109, 110, respectively, so as to be formed in rectangular waveforms. And the demodulation is carried out from thus obtained two signals.
Furthermore in an offset system of FIG. 22, the carrier wave signal and the local oscillator signal having a frequency being offset from the carrier wave signal frequency are mixed together to obtain a base-band signal. The waveform of the base-band signal is waveform-shaped by the low-pass filter 107 and the limiter amplifier 109. And, the demodulation is carried out by discriminating its frequency.
However, in such a conventional system, since the quadrature demodulation system requires two pairs of mixers, channel filters, and limiting amplifiers, it is disadvantageous in size and electric power consumption. On the other hand, in case of the offset demodulation system, since it requires one set of mixer, channel filter, and limiting amplifier, saving electric power consumption is possible. However, when the frequency of the local oscillator signal is equal or close to the frequency of the carrier wave, its frequency difference cannot be detected from an output baseband signal and therefore the demodulation cannot be executed.
Accordingly, the conventional systems have problems such that the local oscillator circuit is required a high frequency stability, or a means such as an automatic frequency controller is required.